Future wireless networks are expected to support massive connections for application scenarios such as Internet-of-Things (IoT) where the devices can be electronic tablets, sensors, wearables, and so on. This thus requires a scalable and efficient solution for providing them perpetual power supply, particularly to achieve the envisioned sustainable and green IoT. To this end, far-field radio-frequency (RF) transmission enabled wireless power transfer (WPT) has recently gained an upsurge of interest [1]}, [2]}, due to its greater convenience as well as larger charging distance than conventional battery replacement and inductive/magnetic resonance coupling based wireless charging techniques. However, energy receivers generally require much higher receive signal power than information receivers, due to their different receiver sensitivities and design objectives in practice.
As such, the low efficiency of WPT for energy receivers over long transmission distances fundamentally limits the performance of practical WPT systems. Although exploiting
the large array/beamforming gain brought by deploying massive antennas at the WPT transmitter can in principle boost the WPT efficiency significantly, it faces various challenges in practical implementation, e.g., exceedingly high energy consumption and hardware cost [3]}, [2]}.
